Transcript Electricity

Electricity
Charge
Defn: Property of matter that determines how it will interact
with other matter via the electrostatic force; opposite charges
attract, like repel
All atoms are composed of subatomic
particles that contain charge; equal
amounts of + and - results in neutrality
Designation of positive and negative is
completely arbitrary
If like charges repel and unlike attract, what holds protons and
neutrons together?
Electrostatic Force
• The force of repulsion depends
upon the size of the charges
and the distance between them
• Force is linear in charge
• Force is proportional to the inverse square of
the distance
F=
k Q1 Q2
r2
Metals
• For current, need charged particles that are
readily given up by atom
• Protons are tightly bound in nucleus
• Metals are atoms
that have outer
electrons that
are not tightly
bound
Current
• Current – time rate of flow of
net charge particles past a
given point
• In order to have a current, need
a) source of charged particles
b) a conducting path
Electric Potential
• An electric field is created by any separation of
charges
• Needed to accelerate/move electrons
• Electrical potential energy related to electric
field
• E.P.E. = Q E d
(Note: G.P.E. = mgh)
• Electric potential is V = E d
• E.P.E. = V d
Potential Sources
• Batteries – chemical storage unit; metals in
acid or base
• Fuel Cell – similar to battery; chemicals react
with no metals necessary
• Generators – electromagnetic induction;
kinetic energy to electricity
• Photovoltaic Cells – direct creation of
electricity from sunlight (later chapter)
Battery
• Two dissimilar metals in an acid or base
• Negative and positive ions
are able to migrate in solution
• With a completed circuit,
negative ion goes to negative
terminal, releasing election
• At positive terminal, electron combines with
positive ion
Fuel Cell
• Similar to battery in that chemicals are
reacting to produce current
• Reactants are fed on demand
into fuel cell
• Exchange of ions across
semi-permeable membrane
allows electrons to complete
circuit
• Weight is still major issue
Conducting Path
• For current, need path that will conduct
electricity; metals best for this in most cases
• Except for superconductors, some resistance
to current will occur
• Amount of resistance depends on a) length,
b) cross sectional area, and c) resistivity
rL
R=
A
Ohm’s Law
• The amount of current flowing through a
device depends on the potential and
resistance
• V=IR
• As resistance increases,
the amount of current
decreases, and vice versa
Circuits
• Two ways to wire a circuit if
more than one element
• Parallel – each element directly wired to
source; more current needed as add new
elements; 1/R = 1/R1 + 1/R2 + …
• Series – same current flows through each
element; current drops as add new ones, as
resistance increases; R = R1 + R2 + …
Moving Charges
Oersted (1820) discovered that a current
creates a magnetic field
This means a current has a force on a
magnet
Newton’s Third Law means that a
magnet has a force on a moving charge
Direction of force is perpendicular to
velocity and magnetic field
Faraday’s Induction Law
• Faraday (1831) noticed that electric potential
could by induced by changing magnetic field
• If potential induced in closed loop, get a
current
• Size of potential depends upon rate at which
magnetic field changes
Generator
Electric generator: rotate coils of wire perpendicular
to magnetic field
Magnet
Layers of wire coils
How do you make electricity?
Need energy source to do the work of turning the
magnet, causing a flow of current (electricity) in
the wire coils
Front view
Convert the potential energy in
something to kinetic energy to turn
magnet
Options
Water behind a dam
Chemical energy in biomass or
fossil fuels
How do you make electricity?
Most electricity formed by steam turbine electric
generators
Electrical Power
We know that P = DE/Dt from the definition of power
The amount of energy delivered by a current is equal to the
amount of charge delivered times the voltage,
DE = Dq V
But
I = Dq/Dt

P=IV

P = (Dq V )/ Dt
Power Lost
• Resistance of wire between source and
appliance causes loss
• Since the voltage in a resistor is given by
Ohm’s Law (V = IR), the power lost is given by
Plost = I V = I (IR) = I2 R
• This loss accounts for about 10% of all
electricity in the U.S.
Transformers
• Energy loss in lines is decreased by sending
electricity at high voltage/low current over
long distances
• Near homes, the voltage
must be stepped down to
prevent lethal accidents
• Transformer use electromagnetic
induction to do this